GAZA CITY, Gaza Strip -- Prime Minister Ariel Sharon said Tuesday that Israel would keep targeting Hamas militants after killing the group's last two leaders in helicopter missile strikes. In the bloodiest fighting in weeks, meanwhile, Israeli soldiers killed four Palestinians in a gunfight in the Gaza Strip meant to stop rocket attacks on Jewish settlements. The rocket fire came in response to Israel's assassination of Abdel Aziz Rantisi, a militant leader, over the weekend. Rantisi had replaced Hamas founder Sheik Ahmed Yassin, who was killed by Israel last month. "We will fight terror and we will not let up on them. In that way we got rid of the first murderer, and in that way a few days ago we got rid of the second murderer, and that is not the end," Sharon said.

Early Wednesday, Israeli troops and tanks moved into Beit Lahiya, a town in northern Gaza, and surrounded a housing project, residents said. No clashes or casualties were reported.

Tuesday's fighting came as Defense Minister Shaul Mofaz visited an Israeli army position in Gaza, promoting Israel's "unilateral disengagement" plan that includes a pullout from the territory.

Sharon's Likud Party is to vote in a referendum on the plan on May 2.

Sharon and Likud were in the forefront of settlement building for decades, and the "disengagement" plan calls for dismantling settlements in the West Bank and Gaza for the first time. But Likud leaders are lining up behind it, and the rank and file are expected to vote in favor.

Talking to troops at a post on the Gaza-Egypt border, scene of almost daily clashes as the military searches for arms smuggling tunnels, Mofaz said the pullout would reduce friction with the Palestinians.

Even after the withdrawal, he said, the Israeli military "can act freely, while remaining in the area around the Gaza Strip."

Israel has said it would reserve the right to attack militants in Gaza even after its withdrawal.

The clash in northern Gaza appeared to foreshadow the type of sequence Mofaz meant -- Israeli forces entering Gaza to stop Palestinian rocket fire.

The barrage of homemade rockets -- 15 over two days -- was one of the largest in months. The Qassam rockets, not much more than hollow tubes with fuel and small explosive warheads, exploded in two blocs of Israeli settlements, wounding one Israeli and damaging five structures.

On Tuesday, Israeli tanks rumbled into a buffer zone between the settlement of Nissanit in northern Gaza and the Palestinian town of Beit Lahiya. The military said some of the rockets were launched from there.

Hundreds of Palestinians pelted the tanks with rocks and firebombs, and then gunmen joined in. Taking cover behind a mound of sand, they exchanged fire with soldiers on a nearby hill.

Soldiers killed four Palestinians, including at least two gunmen, hospital officials said. A 17-year-old was shot dead as he climbed on a tank, the military said. At least 33 Palestinians were wounded, six critically, hospital officials said. Four soldiers were slightly wounded, the military said.

Palestinian health officials initially said that five Palestinians had been killed but later said that one of the dead men had been listed twice under different names.

The rocket attacks intensified after Israel killed Rantisi and two of his bodyguards in a missile strike Saturday night. Following Rantisi's killing, Hamas vowed to carry out "100 unique reprisals."

Israel said the killings were part of its campaign to weaken Hamas in advance of the proposed pullout.

In the West Bank, soldiers raided the town of Silwad, where the overall Hamas leader, Khaled Mashaal, was born. About 70 people were arrested during house-to-house searches, Palestinian security officials said.

Mashaal lives in Damascus, Syria. Israeli officials have said Mashaal could be targeted by Israel.

The soldiers searched the house of Palestinian Cabinet minister Qadoura Fares in Silwad. "It looks like they're looking for a wanted guy from Hamas," Fares said. "I told them who I was, but they still searched my house."